Method for producing polytetrahydrofuran

ABSTRACT

In a process for the single-stage preparation of polytetrahydrofuran and/or tetrahydrofuran copolymers having a mean molecular weight of from 650 to 5000 dalton by polymerization of tetrahydrofuran over an acid catalyst in the presence of at least one telogen and/or comonomer, the telogen and/or comonomer is added at at least two addition points in different segments of the polymerization apparatus.

The present invention relates to a process for preparingpolytetrahydrofuran or tetrahydrofuran copolymers by polymerization oftetrahydrofuran over an acid catalyst, preferably a heterogeneous acidcatalyst, in the presence of at least one telogen and/or comonomer,wherein the telogen and/or comonomer is added at at least two additionpoints in different segments of the polymerization apparatus.

Polytetrahydrofuran, hereinafter referred to as PTHF and also known aspolyoxybutylene glycol, is used as a versatile intermediate in theplastics and synthetic fibers industries and is employed, inter alia,for producing polyurethane, polyester and polyamide elastomers. Inaddition, it is, like some of its derivatives, a valuable auxiliary inmany applications, for example as dispersant or in the deinking of wastepaper.

PTHF is usually prepared industrially by polymerization oftetrahydrofuran, hereinafter referred to as THF for short, over suitablecatalysts. The addition of suitable reagents enables the length of thepolymer chains to be controlled, and the mean molecular weight can thusbe set to the desired value. Control is achieved by choice of type andamount of the telogen. Such reagents are referred to as chaintermination reagents or “telogens”. Selection of appropriate telogensalso enables functional groups to be introduced at one or both ends ofthe polymer chain.

Thus, for example, the use of carboxylic acids or carboxylic anhydridesas telogens results in formation of the monoesters or diesters of PTHFwhich subsequently have to be converted into PTHF by saponification ortransesterification. These processes are therefore referred to astwo-stage PTHF processes.

Other telogens act not only as chain termination reagents but are alsoincorporated into the growing polymer chain of the PTHF. They have notonly the function of a telogen but simultaneously act as a comonomer andcan therefore be referred to as either telogens or comonomers with equaljustification. Examples of such comonomers are telogens having twohydroxy groups, e.g. diols (dialcohols). These can be, for example,ethylene glycol, propylene glycol, butylene glycol, 1,3-propanediol,1,4-butanediol, 2-butyne-1,4-diol, 1,6-hexanediol or low molecularweight PTHF. Further suitable comonomers are cyclic ethers such as1,2-alkylene oxides, for example, ethylene oxide or propylene oxide,2-methyltetrahydrofuran or 3-methyltetrahydrofuran. The use of suchcomonomers leads, with the exception of water, 1,4-butanediol and lowmolecular weight PTHF, to formation of tetrahydrofuran copolymers,hereinafter referred to as THF copolymers, and in this way makes itpossible to achieve chemical modification of PTHF.

In industry, use is predominantly made of two-stage processes in whichtetrahydrofuran is, for example, polymerized in the presence offluorosulfonic acid to form polytetrahydrofuran esters and these aresubsequently hydrolyzed to polytetrahydrofuran. As an alternative,tetrahydrofuran is, for example, polymerized with acetic anhydride inthe presence of acid catalysts to form polytetrahydrofuran diacetatewhich is subsequently transesterified, e.g. by means of methanol, togive polytetrahydrofuran. Disadvantages of such processes are that theyhave to be carried out in two stages and that by-products such ashydrofluoric acid and methyl acetate are formed.

The single-stage synthesis of PTHF is carried out by polymerization ofTHF using water, 1,4-butanediol or low molecular weight PTHF as telogenover acid catalysts. Known catalysts include both systems which arehomogeneously dissolved in the reaction system and heterogeneous, i.e.largely undissolved, systems.

It is an object of the present invention to provide an economicalprocess by means of which polytetrahydrofuran and/or tetrahydrofurancopolymers having a particular mean molecular weight can be prepared inhigher polymer yields and/or space-time yields.

We have found that this object is achieved by a process for preparingpolytetrahydrofuran (PTHF) and/or tetrahydrofuran copolymers (THFcopolymers) by polymerization of tetrahydrofuran over a heterogeneousacid catalyst in the presence of at least one telogen and/or comonomer,wherein the telogen and/or comonomer is added at at least two additionpoints in different segments of the polymerization apparatus.

The process of the present invention makes it possible to obtain PTHFand THF copolymers having a particular mean molecular weight in highspace-time yield and at a high conversion, with the process of thepresent invention being able to be carried out in one stage or in twostages. However, preference is given to the single-stage synthesis ofPTHF.

It has surprisingly been found that the cascaded addition of the telogenand/or comonomer at at least two different addition points in differentsegments of the polymerization apparatus enables the space-time yieldand conversion to be improved significantly. The number of additionpoints can be two, three, four, five or more and depends on thepolymerization apparatus used, in particular its type and capacity, andalso on process engineering and economic boundary conditions. However,preference is generally given to using from 2 to 5 addition points.

Examples of suitable polymerization apparatuses are cascades of at leasttwo tank or tube reactors, for example cascades of stirred tanks,cascades of at least two fixed-bed reactors, which may optionally beoperated with circulation, and cascades of loop reactors. In thesepolymerization apparatuses, one segment in which an addition point forthe telogen and/or comonomer is located corresponds to a tank or a tube.However, it is not necessary for each segment of the polymerizationapparatus in which an addition point for the telogen and/or comonomer islocated to be a single unit such as a stirred tank. Rather, a reactorcan be configured so that it fulfills the function of a plurality ofreactor elements connected in series. It is therefore also possible touse a single reactor, in particular a fixed-bed reactor, which isdivided into at least two, preferably from 2 to 5, segments by means ofsuitable internals, for example orifice plates or sieve trays.Furthermore, it is possible to use stirred columns having more than onestage and flow tubes, each having at least two addition points.Particular preference is given to cascades of stirred tanks comprisingat least two stirred tanks, preferably from 2 to 5 tanks.

The telogen can be introduced into the polymerization either separatelyon its own or as a solution in the THF, with preference being given to atelogen content of from 1 to 50 mol %, based on tetrahydrofuran.Comonomers can likewise be introduced into the polymerization assolutions in THF, in which case the comonomer content can be up to 30mol %, preferably 20 mol %, based on tetrahydrofuran. However, the THFand the telogen and/or comonomer are preferably introduced separatelyinto the polymerization reactor.

Since the telogen effects termination of the polymerization, the meanmolecular weight of the PTHF or the THF copolymers can be controlled viathe amount of telogen used. The more telogen present in the reactionmixture, the lower the mean molecular weight of the resulting PTHF orTHF copolymers. Depending on the telogen content of the polymerizationmixture, it is possible to prepare PTHF and THF copolymers having meanmolecular weights of from 650 to 5 000 dalton, preferably from 650 to 3000 dalton and particularly preferably from 1 000 to 3 000 dalton.

The amount of telogen and/or comonomer added at each of the two or moreaddition points can be identical or different. At least 5% by weight ofthe total amount of telogen and/or comonomer, preferably at least 10% byweight, particularly preferably at least 15% by weight, are added afterthe first addition point, i.e. at the second addition point ordistributed over the second to n-th addition points. Consequently, theaddition at the beginning of the polymerization unit, i.e. at the firstaddition point, is not more than 95% by weight, preferably not more than90% by weight and particularly preferably not more than 85% by weight.

The addition of the telogen and/or comonomer can be controlled bysetting the mean molecular weight of the PTHF or THF copolymer in eachsegment of the polymerization apparatus, in the case of a cascade ofstirred tanks, in each tank, via the amount of telogen and/or comonomeradded in this segment so that this mean molecular weight corresponds tothe mean molecular weight to be achieved for the end product. To achievethis, telogen and/or comonomer is introduced empirically, the meanmolecular weight of a sample is determined in a known manner, and thetelogen and/or comonomer addition is altered as a function of the resultof this determination.

Apart from control via the mean molecular weight, the amount of telogenand/or comonomer can also be controlled so that each segment displaysthe same incremental productivity based on the amount of catalyst. Theproductivity (“prod”) is calculated, as described below in the presentpatent application, from throughput, catalyst content and differencebetween evaporation residue (“ER”) at the inlet and outlet of a segment.The evaporation residue can be determined with sufficient accuracy fromintrinsic properties of the feed stream and the exit stream, preferablyfrom the index of refraction, with the aid of a calibration curve. Inthe present context, the feed stream is the stream of reaction mixtureintroduced into the segment and the exit stream is the stream ofreaction mixture leaving the segment. Control of the telogen and/orcomonomer addition both via the mean molecular weight and via theincremental productivity requires repeated sampling, determinations and,as a function of their results, changes to the addition.

If continuous control by means of the mean molecular weight or theincremental productivity is considered too complicated, it is alsopossible to determine a value T for the amount of telogen and/orcomonomer/segment of the polymerization apparatus for the respectivedesired end product once for each segment by means of the mean molecularweight or the incremental productivity and then to add from 25 to 400%of T, preferably from 33 to 300%, particularly preferably from 50 to200%, at each addition point. In this case, the amount of telogen and/orcomonomer added can be identical or different at each of the variousaddition points.

The physical control of the telogen and/or comonomer addition in thevarious segments of the polymerization apparatus is carried out in amanner known per se by means of individual pumps, valves, nozzles,orifice plates, slits, membranes, filter plates or capillary tubes.

Suitable telogens and/or comonomers in the preferred single-stageprocess of the present invention are saturated or unsaturated,unbranched or branched alpha, omega-C₂-C₁₂-diols, water,polytetrahydrofuran having a molecular weight of from 200 to 700 dalton,cyclic ethers or mixtures thereof. Preference is given to water,ethylene glycol, 1,3-propanediol, 1,4-butanediol, 1,5-pentanediol,1,6-hexanediol, polytetrahydrofuran having a molecular weight of from200 to 700 dalton, 1,6-hexanediol, 1,8-octanediol, 1,10-decanediol,2-butyne-1,4-diol and neopentyl glycol or mixtures thereof, withparticular preference being given to water, 1,4-butanediol and/orpolytetrahydrofuran having a molecular weight of from 200 to 700 dalton.

Telogens used for preparing PTHF and THF copolymers by the two-stagemethod are carboxylic anhydrides and/or carboxylic anhydride/protic acidmixtures. Preferred carboxylic anhydrides, which are derived fromaliphatic or aromatic polycarboxylic or monocarboxylic acids, containfrom 2 to 12, preferably from 1 to 8, carbon atoms. Particularpreference is given to acetic anhydride. The protic acids are preferablyorganic and inorganic acids which are soluble in the reaction system.Preferred carboxylic acids are aliphatic or aromatic polycarboxylicand/or monocarboxylic acids containing from 2 to 12, preferably from 1to 8, carbon atoms. Examples of aliphatic carboxylic acids are aceticacid, lactic acid, propionic acid, valeric acid, caproic acid, caprylicacid and pelargonic acid. Examples of aromatic carboxylic acids arephthalic acid and naphthalenecarboxylic acid. Among these carboxylicacids, preference is given to using acetic acid.

Suitable comonomers are cyclic ethers which can be polymerized withopening of the ring, preferably three-membered, four-membered andfive-membered rings such as 1,2-alkylene oxides, for example ethyleneoxide or propylene oxide, oxetane, substituted oxetanes such as3,3-dimethyloxetane, and the THF derivatives 2-methyltetrahydrofuran and3-methyltetrahydrofuran. Particular preference is given to2-methyltetrahydrofuran or 3-methyltetrahydrofuran.

As polymerization catalysts, it is possible to use homogeneous acidcatalysts and heterogeneous acid catalysts. Among the suitablehomogeneous catalysts, particular mention may be made ofheteropolyacids.

Examples of heteropolyacids which can be used as catalysts in theprocess of the present invention are the following compounds:

-   -   dodecamolybdophosphoric acid (H₃PMo₁₂O₄₀*n H₂O),    -   dodecamolybdosilicic acid (H₄SiMo₁₂O₄₀*n H₂O),    -   dodecamolybdoceric(IV) acid (H₈CeMo₁₂O₄₂*n H₂O),    -   dodecamolybdoarsenic(V) acid (H₃AsMo₁₂O₄₂*n H₂O),    -   hexamolybdochromic(III) acid (H₃CrMo₆O₂₄H₆*n H₂O),    -   hexamolybdonickelic(II) acid (H₄NiMo₆O₂₄H₆*5 H₂O),    -   hexamolybdoiodic acid (H₅JMo₆O₂₄*n H₂O),    -   octadecamolybdodiphosphoric acid (H₆P₂Mo₁₈O₆₂*11 H₂O),    -   octadecamolybdodiarsenic(V) acid (H₆As₂Mo₁₈O₆₂*25 H₂O),    -   nonamolybdomanganic(IV) acid (H₆MnMo₉O₃₂*n H₂O),    -   undecamolybdovanadophosphoric acid (H₄PMo₁₁VO₄₀*n H₂O),    -   decamolybdodivanadophosphoric acid (H₅PMo₁₀V₂O₄₀*n H₂O),    -   dodecavanadophosphoric acid (H₇PV₁₂O₃₆*n H₂O),    -   dodecatungstosilicic acid (H₄SiW₁₂O₄₀*7 H₂O),    -   dodecatungstoboric acid (H₅BW₁₂O₄₀*n H₂O),    -   octadecatungstodiphosphoric acid (H₆P₂W₁₈O₆₂*14 H₂O),    -   octadecatungstodiarsenic(V) acid (H₆As₂W₁₈O₆₂*14 H₂O),    -   hexamolybdohexatungstophosphoric acid (H₃PMo₆W₆O₄₀*n H₂O).

Of course, it is also possible to use mixtures of heteropolyacids. Dueto their ready availability, preference is given to usingdodecatungstophosphoric acid, dodecamolybdophosphoric acid,nonamolybdophosphoric acid, dodecamolybdosilicic acid anddodecatungstosilicic acid in the process of the present invention.

According to the present invention, preference is given to using thefree heteropolyacids as catalysts, but it is also possible to utilizetheir salts, in particular their alkali metal and alkaline earth metalsalts, as catalysts. The heteropolyacids and their salts are knowncompounds and can be prepared by known methods, for example by themethods described in Brauer (Editor): Handbuch der PräparativenAnorganischen Chemie, Volume III, Enke, Stuttgart, 1981 or the methodsdescribed in Top. Curr. Chem. 76, 1 (1978).

The heteropolyacids prepared by these methods are generally in hydratedform. They are preferably freed of the coordinated water present thereinbefore they are used in the process of the present invention. Thisdehydration can advantageously be carried out thermally, for example bythe method described in Makromol. Chem. 190, 929 (1989).

Preferred polymerization catalysts are heterogeneous acid catalystswhich contain acid centers having an acid strength H₀ of <+2 in aconcentration of at least 0.005 mmol/g of catalyst, particularlypreferably an acid strength H₀ of <+1.5 in a concentration of at least0.01 mmol/g of catalyst.

Heterogeneous polymerization catalysts which can be used in the processof the present invention are supported heteropolyacids, cesium salts ofheteropolyacids, sulfate- or phosphate-doped metal oxides of groups IVA,VIIA and VIIIA of the Periodic Table of the Elements, if desiredacid-activated sheet silicates or zeolites, polymers comprisingalpha-fluorosulfonic acids, mixed acid metal oxides of groups IIIB, IVB,IIIA to VIIIA of the Periodic Table of the Elements, supported catalystscomprising an oxidic support material and a catalytically active amountof a tungsten or molybdenum compound or mixtures of such compounds, withpreference being given to supported catalysts comprising an oxidicsupport material and a catalytically active amount of a tungsten ormolybdenum compound or mixtures of such compounds.

Suitable supported heteropolyacids are, for example, heteropolyacids ofthe formulae H₃PM₁₂O₄₀ (dodecamolybdophosphoric acid,dodecatungstophosphoric acid) and H₃SiM₁₂O₄₀ (dodecamolybdosilicic acid,dodecatungstosilicic acid), where M is molybdenum and/or tungsten, whichare supported on customary support materials such as activated carbon,silicon dioxide, on mesoporous silicon oxides grafted with sulfonicacids, as are obtained by treatment of the mesoporous silicon oxidesupports with mercaptoalkyltrialkoxysilanes and subsequent oxidation,for example using hydrogen peroxide or nitric acid, clays, polymers orzeolites such as MCM-41. Such supported heteropolyacids are described,for example, in T. Okuhara, N. Miszuno and M. Misono, CatalyticChemistry of Heteropoly Compound, Adv. Catal. 41, 1996, Acad. Press,page 113 ff. The acid cesium salts of the heteropolyacids described inthis reference, for example Cs_(2.5)H_(0.5)PW₁₂O₄₀, are also suitablefor use as polymerization catalyst in the process of the presentinvention.

As sulfate-doped metal oxides of groups IVA, VIIA and VIIIA, preferenceis given to sulfate-doped zirconium dioxide and titanium dioxide,sulfate-doped mixed oxides of iron (III) and zirconium dioxide and alsosulfate- or phosphate-doped mixed oxides of manganese and zirconiumdioxide. The preparation of these sulfate-doped metal oxides is carriedout by methods known per se. Thus, sulfate-doped zirconium dioxide whichis suitable for the process of the present invention can be prepared,for example, by the process described in U.S. Pat. No. 5,149,862.

Apart from sulfate-doped metal oxides, it is also possible to usepolymers comprising alpha-fluorosulfonic acid as polymerizationcatalyst. Preference is given to perfluorinated polymers comprisingalpha-fluorosulfonic acid which are marketed, for example, under thetrade name Nafion® by E. I. du Pont de Nemours and Company and under thetrade names Amberlyst® 15 and Amberlyst® 36 by Rohm and Haas.

Furthermore, it is possible to use mixed acid metal oxides of groupsIIIB, IVB, IIIA to VIIIA of the Periodic Table of the Elements, inparticular WO₃—TiO₂, WO₃—ZrO₂, WO₃—SnO₂, MoO₃—TiO₂, MoO₃—ZrO₂,MoO₃—SnO₂, V₂O₅—WO₃—TiO₂, TiO₂—SiO₂, ZrO₂—SiO₂, Al₂O₃—SiO₂, as aredescribed in K. Tanabe, M. Misono, Y. Ono and H. Hattori, New Solidacids and Bases, Stud. Surf. Sciences and Catal. 51, in particular pages108–128 and 199–210, Elsevier 1989, in the process of the presentinvention.

Suitable supported catalysts which comprise an oxidic support materialand oxygen-containing molybdenum or tungsten compounds or mixtures ofsuch compounds as catalytically active compounds and may also, ifdesired, be additionally doped with sulfate or phosphate groups aredescribed in DE-A 44 33 606, which is hereby expressly incorporated byreference. These catalysts can be pretreated with a reducing agent,preferably with hydrogen, as described in DE 19641481 which is herebyexpressly incorporated by reference.

Further suitable catalysts are the supported catalysts described in theGerman patent application DE 19649803, which is hereby likewiseexpressly incorporated by reference, which comprise a catalyticallyactive amount of at least one oxygen-containing molybdenum and/ortungsten compound as active composition and have been calcined at from500° C. to 1 000° C. after application of the precursor compounds of theactive composition to the support material precursor, and furthercomprise a promoter which comprises at least one element or a compoundof an element of group 2, 3 including the lanthanides, 5, 6, 7, 8 or 14of the Periodic Table of the Elements. These catalysts generally containfrom 0.01 to 30% by weight, preferably from 0.05 to 20% by weight andparticularly preferably from 0.1 to 15% by weight, of promoter,calculated as the sum of its constituents in the form of their elementsand based on the total weight of the catalyst.

The catalysts which are known from DE-A 44 33 606 and DE 196 49 803 andcan be employed according to the present invention generally containfrom 0.1 to 50% by weight of the catalytically active, oxygen-containingcompounds of molybdenum or tungsten or of mixtures of the catalyticallyactive, oxygen-containing compounds of these metals, in each case basedon the total weight of the catalyst and, since the chemical structure ofthe catalytically active, oxygen-containing compounds of molybdenumand/or tungsten is not known precisely, in each case calculated as MoO₃and/or WO₃.

The German patent application DE 10032267.0 “Katalysator und Verfahrenzur Herstellung von Polytetrahydrofuran” describes catalysts which canbe employed according to the present invention and comprise at least onecatalytically active, oxygen-containing molybdenum and/or tungstencompound on an oxidic support and in which the content of molybdenumand/or tungsten, based on the catalyst dried under nitrogen at 400° C.,is x μmol of (tungsten and/or molybdenum)/m² of surface area, where10.1<x<20.9. The catalyst activity was able to be increasedsignificantly by targeted setting of the ratio of the tungsten and/ormolybdenum content to the BET surface area.

Furthermore, the German patent application DE 10032268.9 “VerbesserterKatalysator und Verfahren zur Herstellung von Polytetrahydrofuran” filedon the same day describes catalysts which can be used according to thepresent invention and comprise at least one catalytically active,oxygen-containing molybdenum and/or tungsten compound on an oxidicsupport and have been calcined at from 400° C. to 900° C. afterapplication of the precursor compounds of the catalytically activecompounds to the support material or a support material precursor. Thesecatalysts are porous and contain transport pores having a diameter of<25 nm and have a volume of these transport pores of at least 50 mm³/g.

The catalysts described in these two applications contain from 0.1 to70% by weight, preferably from 5 to 40% by weight and particularlypreferably from 10 to 35% by weight, of the catalytically activeoxygen-containing molybdenum and/or tungsten compound(s), calculated asMoO₃ and/or WO₃ and based on the total weight of the catalyst.

Suitable oxidic supports for the catalysts comprising oxygen-containingmolybdenum or tungsten compounds or mixtures of such compounds ascatalytically active compounds are, for example, zirconium dioxide,titanium dioxide, hafnium oxide, yttrium oxide, iron(III) oxide,aluminum oxide, tin(IV) oxide, silicon dioxide, zinc oxide or mixturesof these oxides. Particular preference is given to zirconium dioxide,titanium dioxide and/or silicon dioxide, in particular titanium dioxide.

Apart from the abovementioned polymerization catalysts, it is possibleto use sheet silicates or zeolites which may, if desired, have beenactivated by acid treatment as heterogeneous catalysts in the process ofthe present invention. As sheet silicates, preference is given to usingthose of the montmorillonite-saponite, kaolin-serpentine orpalygorskite-sepiolite group, particularly preferably montmorillonite,hectorite, kaolin, attapulgite, beiddellite or sepiolite, as aredescribed, for example, in Klockmanns Lehrbuch der Mineralogie, 16^(th)edition, F. Euke Verlag 1978, pages 739–765.

In the process of the present invention, it is possible to use, forexample, the montmorillonites available under the trade names Tonsil®,Terrana® and Granosil or as catalysts of the types Tonsil® K 10, KSF-O,KO or KS from Süd-Chemie AG, Munich. Attapulgites suitable for use inthe process of the present invention are, for example, marketed byEngelhard Corporation, Iselin, USA, under the trade names Attasorb® RVMand Attasorb® LVM.

The term zeolites refers to a class of aluminum hydrosilicates which,owing to their particular chemical structure, have a three-dimensionalnetwork with defined pores and channels in the crystal. Both natural andsynthetic zeolites are suitable for the process of the presentinvention, with preference being given to zeolites having an SiO₂—Al₂O₃molar ratio of from 4:1 to 100:1, particularly preferably an SiO₂—Al₂O₃molar ratio of from 6:1 to 90:1 and very particularly preferably anSiO₂—Al₂O₃ molar ratio of from 10:1 to 80:1. The primary crystallites ofthe zeolites preferably have a size of up to 0.5 mm, preferably up to0.1 mm and particularly preferably up to 0.05 mm.

The zeolites which can be used in the process of the present inventionare used in the H form. In this form, acidic OH groups are present inthe zeolite. If the zeolites are not obtained directly in the H form inthe process for producing them, they can easily be converted into thecatalytically active H form by acid treatment using, for example,mineral acids such as hydrochloric acid, sulfuric acid or phosphoricacid or by thermal treatment of suitable precursor zeolites which, forexample, contain ammonium ions, for example by heating to from 450 to600° C., preferably from 500 to 550° C. Examples of zeolites which canbe employed according to the present invention are zeolites of themordenite type, beta-zeolites, zeolites of, for example, the MCM-22 typeor Faujasite.

The heterogeneous catalysts which can be employed according to thepresent invention can be used in the form of powders, for example whenthe process is carried out by the suspension method, or advantageouslyas shaped bodies, e.g. in the form of cylinders, spheres, rings, spiralsor granules, particularly when a fixed bed of catalyst is used in theprocess of the present invention.

As monomer, it is possible in principle to use any THF, preferably THFcontaining less than 100 ppm of unsaturated compounds and molecularoxygen. Preference is given to using commercially available THF whichhas been prepurified by acid treatment, for example as described in EP-A003 112, or by distillation.

The polymerization is generally carried out at from 0 to 80° C.,preferably from 25 to 75° C. and particularly preferably from 40 to 70°C. The pressure employed is generally not critical for the result of thepolymerization, which is why the process is generally carried out atatmospheric pressure or under the autogenous pressure of thepolymerization system.

To avoid formation of ether peroxides, the polymerization isadvantageously carried out under an inert gas atmosphere. Inert gaseswhich can be used are, for example, nitrogen, carbon dioxide or noblegases; preference is given to using nitrogen.

The polymerization can also be carried out in the presence of hydrogenat hydrogen pressures of from 0.1 to 10 bar.

The process of the present invention is preferably carried outcontinuously, but it is also possible for it to be carried outbatchwise.

The reaction can be carried out in conventional reactors or reactorassemblies suitable for continuous processes in a suspension orfixed-bed mode, for example in loop reactors or stirred reactors in thecase of a suspension process or in tube reactors or fixed-bed reactorsin the case of a fixed-bed process.

When a continuous polymerization apparatus is used, the catalyst can, ifdesired, be pretreated after it has been introduced into the reactor(s).Suitable pretreatments for the catalyst are, for example, drying bymeans of gases, for example air or nitrogen, which have been heated to80–200° C., preferably 100–150° C., or pretreatment with a reducingagent as is described in DE 196 41 481 for the supported catalysts whichare preferred according to the present invention and comprise, as activecomposition, a catalytically active amount of at least oneoxygen-containing molybdenum and/or tungsten compound. However, thecatalyst can of course also be used without pretreatment.

In a fixed-bed process, the polymerization apparatus can be operated inthe upflow mode, i.e. the reaction mixture is conveyed from the bottomupward, or in the downflow mode, i.e. the reaction mixture is conveyedthrough the reactor from the top downward. The feed mixture comprisingTHF and part of the telogen and/or comonomer is introduced continuouslyat the first addition point of the polymerization apparatus. Furtheramounts of telogen and/or comonomer are added at the second or furtheraddition points. The space velocity is from 0.05 to 0.8 kg of THF/(l·h),preferably from 0.1 to 0.6 kg of THF/(l·h) and particularly preferablyfrom 0.15 to 0.5 kg of THF/(l·h).

Furthermore, the polymerization reactor can be operated in a singlepass, i.e. without product recirculation, or in the circulation mode,i.e. the polymerization mixture leaving the reactor is circulated. Inthe circulation mode, the ratio of recirculated mixture to fresh feed isless than or equal to 100:1, preferably less than 50:1 and particularlypreferably less than 40:1.

EXAMPLES

The invention is illustrated by the examples below.

Molecular Weight Determination

The mean molecular weight M_(n), namely the number average molecularweight defined as the mass of all PTHF molecules divided by their amountin mol, is determined by measurement of the hydroxyl number of thepolytetrahydrofuran. The hydroxyl number is the amount of potassiumhydroxide in mg which is equivalent to the amount of acetic acid boundby 1 g of substance on acetylation. The hydroxyl number is determined byesterification of the hydroxyl groups present by means of an excess ofacetic anhydride.H—[O(CH₂)₄]_(n)—OH+(CH₃CO)₂O→CH3CO—[O(CH₂)₄]_(n)—O—COCH₃+H₂O

After the reaction, the excess acetic anhydride is hydrolyzed with waterin accordance with the following reaction equation(CH₃CO)₂O+H₂O→2CH₃COOHand backtitrated as acetic acid using sodium hydroxide.

Example 1 Production of the Catalyst

Titanium dioxide (VKR 611 from Sachtleben) was calcined at 500° C. in acontinuous rotary tube so as to give a material having a BET surfacearea of 135 m²/g and a loss on ignition of 8.1% by weight. 73.81 kg ofthis titanium dioxide were mixed in a pan mill with 29.64 kg of ammoniumparatungstate, 21.73 kg of oxalic acid dihydrate and 17.7 kg of waterfor 1 hour, extruded to form extrudates having a diameter of 3 mm anddried at 90° C. for 3 hours. The extrudates were subsequently calcinedat 400° C. for 5 hours and at 630° C. for 40 minutes.

Continuous Polymerization of THF

Comparative Examples C 1 to 4

A 3 300 ml atmospheric-pressure glass reactor which was provided with aheating mantle and an external stirrer and in which the region of thestirrer was separated from the remainder of the reactor by means of awire mesh was charged with 2 410 g of catalyst as described inExample 1. Catalyst and reactor were subsequently dried at 200° C. in astream of N₂. After the catalyst had been introduced, the reactor wascooled to 60° C. and started up using a mixture of 1,4-butanediol in THF(tetrahydrofuran). Samples were taken daily at the outlet from thereactor and were freed of unreacted THF and 1,4-butanediol by heating at120° C. at 0.3 mbar.

The evaporation residue obtained in this way corresponds approximatelyto the THF conversion and is defined asER=m(distillation residue)/m(sample).

The productivity is in turn defined asProd=ER*m(feed)/t/m(catalyst)where m is the mass in g, t is the residence time in hours and feedrefers to the reaction mixture fed into the reactor.

The number average molecular weight was then measured on the PTHFobtained in this way. The amount of butanediol fed in was subsequentlyvaried until the number average molecular weight of the PTHF was 2000±50 on three successive days. The evaporation residue and themolecular weight of the PTHF were then determined. The results are shownin table 1.

Comparative Example C 5

602.5 g of catalyst as described in Example 1 were placed in eachreactor of a cascade of four of the atmospheric-pressure glass reactorsdescribed for comparative examples 1 to 4 and the catalyst and reactorswere subsequently dried at 200° C. in a stream of N₂. The reactors wereoperated as a cascade of stirred tanks. After the catalyst had beenintroduced, the reactors were cooled to 60° C. and started up using amixture of 1,4-butanediol in THF (tetrahydrofuran). 100% of the telogen1,4-butanediol was added upstream of the first reactor. Samples weretaken daily at the outlet from the last reactor of the cascade and werefreed of unreacted THF and 1,4-butanediol by heating at 120° C. at 0.3mbar.

The number average molecular weight of the PTHF obtained was thenmeasured and the evaporation residue after the fourth reactor wasdetermined. The number average molecular weight of the PTHF after thelast reactor was 1 989 g/mol. The results and the total throughputs areshown in table 1.

TABLE 1 Comparative examples C 1 to C 5 THF 1,4-BDO¹ Comparativethroughput throughput M_(n) ² ER³ Prod⁴ examples (g/h) (g/h) (g/mol) (%)(g/kg/h) C 1 1 000  3.4 1 981 4.6 19.2 C 2 498 2.31 2 034 6.8 14.2 C 3332 1.82 2 021 8.3 11.5 C 4 249 1.53 1 993 9.6 10.0 C 5 499 2.09 1 9896.7 14.0 ¹= 1,4-Butanediol ²= Number average molecular weight ³=Evaporation residue ⁴= Productivity

Examples 2 to 7

602.5 g of catalyst as described in Example 1 were placed in eachreactor of a cascade of four of the atmospheric-pressure glass reactorsdescribed for comparative examples 1 to 4 and the catalyst and reactorswere subsequently dried at 200° C. in a stream of N₂. The reactors wereoperated as a cascade of stirred tanks. After the catalyst had beenintroduced, the reactors were cooled to 60° C. and started up using amixture of 1,4-butanediol in THF (tetrahydrofuran). Upstream of eachreactor unit of the cascade of stirred tanks, the indicated proportionsof the total amount of 1,4-butanediol were fed in by means of a meteringpump. The amount was in each case determined by means of an analyticalbalance. Samples were taken daily at the outlet from the last reactor ofthe cascade and were freed of unreacted THF and 1,4-butanediol byheating at 120° C. at 0.3 mbar.

The number average molecular weight of the PTHF obtained in this way wasthen measured. The amounts of butanediol metered in were subsequentlyvaried until the number average molecular weight of the PTHF after thelast reactor was 2000±50 on three successive days. The evaporationresidue and the molecular weight of the PTHF after each reactor werethen determined. The results for the individual reactors are reportedafter each example below. The results for the fourth reactor and thetotal throughputs are shown in table 3.

Example 2

In Example 2, 25% by weight of the total amount of 1,4-butanediol wereadded upstream of the first reactor. Table 2 shows the total throughputsand the number average molecular weight, the evaporation residue ERafter the last reactor and the productivity.

Reactor No. ER¹ (%) M_(n) ² (g/mol) 1 5.2 3 384 2 6.6 2 631 3 7.1 2 2594 7.6 2 026 ¹= Evaporation residue ²= Number average molecular weight

Example 3

In the cascade of stirred vessels operated in example 3, 1,4-butanediolwas added in four parts, namely 40% by weight upstream of the firstreactor, 30% by weight before the second reactor, 20% by weight beforethe third reactor and 10% by weight before the fourth reactor. Table 2shows the total throughputs and the number average molecular weight, theevaporation residue ER after the last reactor and the productivity.

Reactor ER¹ (%) M_(n) ² (g/mol) 1 3.8 2 521 2 5.5 2 187 3 6.7 2 053 47.9 2 012 ¹= Evaporation residue ²= Number average molecular weight

Examples 4, 5, 6 and 7

In the cascades of stirred vessels operated in Examples 4, 5, 6 and 7,1,4-butanediol was added in four parts at different total throughputs,namely 50% by weight upstream of the first reactor, 25% by weight beforethe second reactor, 15% by weight before the third reactor and 10% byweight before the fourth reactor.

Example Reactor No. ER (%) M_(n) (g/mol) 4 1 2.6 1 805 2 4.4 1 811 3 6.11 892 4 7.9 2 007 5 1 1.5 1 729 2 2.7 1 758 3 3.9 1 869 4 5.2 2 011 6 13.5 1 874 2 5.8 1 859 3 7.8 1 907 4 9.9 2 005 7 1 4.2 1 873 2 6.9 1 8713 9.3 1 941 4 11 2 006

Table 2 shows the total throughputs, the number average molecularweight, the evaporation residue ER after the last reactor and theproductivity.

TABLE 2 Examples 2 to 7 Total Total THF 1,4-BDO¹ throughput throughputM_(n) ² ER³ Prod⁴ Examples (g/h) (g/h) (g/mol) (%) (g/kg/h) 2 496 3.92 2026 7.6 15.8 3 498 2.78 2 012 7.9 16.4 4 498 2.53 2 007 7.9 16.4 5 1000  3.64 2 011 5.2 21.7 6 332 2.03 2 005 9.9 13.7 7 248 1.72 2 006 11.512.0 ¹= 1,4-Butanediol ²= Number average molecular weight ³= Evaporationresidue ⁴= Productivity

Comparison of the examples according to the present invention with thecomparative examples in which the THF throughputs were comparable showsthat higher productivities can be achieved by means of the process ofthe present invention.

1. A process for the single-stage preparation of polytetrahydrofuranand/or tetrahydrofuran copolymers having a mean molecular weight of from650 to 5000 dalton by polymerization of tetrahydrofuran over an acidcatalyst in the presence of at least one telogen and/or comonomer,wherein the telogen and/or comonomer is added at at least two additionpoints in different segments of the polymerization reactor or cascade ofat least two polymerization reactors.
 2. A process as claimed in claim1, wherein a heterogeneous acid catalyst is used.
 3. A process asclaimed in claim 1, wherein telogen and/or comonomer is added at from 2to 5 addition points in different segments of the polymerization reactoror cascade of at least two polymerization reactors.
 4. A process asclaimed in claim 1, wherein a segment of the cascade of polymerizationreactors is a tank reactor or tube reactor.
 5. A process as claimed inclaim 1, wherein the polymerization reactor is a single reactor which isdivided into segments by means of suitable internals.
 6. A process asclaimed in claim 1, wherein at least 5% by weight of the telogen and/orcomonomer are added after the first addition point.
 7. A process asclaimed in claim 1, wherein the control parameter employed for theaddition of the telogen and/or comonomer in the individual segments isthe mean molecular weight of the PTHF or THF copolymer or theproductivity in these segments.
 8. A process as claimed in claim 1,wherein polytetrahydrofuran is prepared from tetrahydrofuran and1,4-butanediol, water and/or polytetrahydrofuran having a molecularweight of from 200 to 700 dalton as telogen.